1. Field of the Invention
The present invention relates to an acousto-optic modulation device.
2. Description of Background Information
An example of the structure of acousto-optic modulation device (referred to as A/O modulator hereinafter) that is a sort of light modulator, is illustrated in FIG. 1. As shown in the figure, an oscillator element 32 made of a piezoelectric element for example is provided on an end surface of a solid block 31 of a transparent medium such as quartz glass. A high-frequency voltage having a magnitude corresponding to the signal level of a modulation signal is applied to the oscillator element 32 by means of a high-frequency oscillator 33.
In the A/O modulator having the structure described above, ultrasonic vibrations are generated in the solid block 31 by the application of the high-frequency voltage to the oscillator element 32. By these ultrasonic vibrations, high and low density portions appear in the medium. As a result, the distribution of refractive index for light changes periodically, forming a laminar distribution. This change of refractive index, on the other hand, functions as a phase type diffraction lattice, to diffract the incident light, whose direction is parallel to the wavefront of the ultrasonic wave entering sideways, into several directions. Especially, under the Bragg's condition, diffraction lights of higher orders are canceled by the effect of interference, so that there remain only first order diffraction lights. In this way, the incident light is divided into a zero order light and the first order lights.
When this A/O modulator is used, for example, for modulating a laser light in a laser projection type display unit, normally the first order light is used as the modulation light because the coefficient of extinction can be made larger for the first order light than the zero order light. The term "coefficient of extinction" represents the ratio Imin/Imax (or its inverse number) between the minimum value Imin and the maximum value Imax of the intensity of output light in the light intensity modulation.
However, if the first order light is used as the modulation light, a problem arises that the efficiency of diffraction is not sufficiently high even though a high value of the coefficient of extinction can be attained. Theoretically, the efficiency of diffraction can be raised by making the length L of the oscillation element 32 longer. This, however, is a difficult requirement in view of manufacturing techniques.